1. Field of the Invention
The present invention relates to a main bearing structure for mounting a crankshaft of a reciprocating internal combustion engine.
2. Disclosure Information
During the manufacturing of the crankcase or cylinder block of an internal combustion engine, it is necessary that the main bearing caps, which mount the crankshaft of the engine be bored and finished at the same time the main bearing structure of the cylinder block is bored and finished. Accordingly, semi-finished main bearing caps and cylinder block casting are bolted together and finished either by means of a boring bar, broaching, or by some other means. Once the main bearing bores are finished, the caps must be removed and bearing inserts put in place. Typically, one bearing insert will be placed in the block in each main bearing cap facing location. Then, the crankshaft is put in contact with the main bearing shells in the cylinder block and the main bearing caps, having another set of bearing shells, are placed down upon the crankshaft. Thereafter, the main bearing caps are screwed into place. Needless to say, it is necessary that the main bearing caps and cylinder block be maintained in very precise alignment, so as to prevent problems such as premature bearing wear, bearing scuffing, overheating, and engine failure.
For many years, engines have featured cylinder blocks with deep skirt designs providing a pocketing effect for main bearing caps. This pocketing involves bringing the block-to-bearing cap mating surface down around the sides of the main bearing caps. Although this arrangement proved very satisfactory when iron main bearing caps and cylinder blocks were used, it has much less importance with the advent of aluminum cylinder blocks. With aluminum cylinder blocks, it has been increasingly known to use a ladder, or bearing beam or girdle assembly in which all of the main bearing caps are consolidated into a single ladder frame.
Although ladder frames offer the advantage of additional rigidity and noise control because they rigidly bond the main bearing caps together, it is not possible to use a pocketed type of design for the caps. Indeed, with aluminum cylinder blocks, pocketed design would perhaps not be advantageous. As a result, the ability to repeatedly install the main bearing caps upon the cylinder block has suffered.
Engine designers seeking to obtain and maintain correct alignment between main bearing caps in the form of a ladder frame used with light metal cylinder blocks have increasingly turned to the use of dowels which extend between the cylinder block and the main bearing beam assembly. Such dowels have been applied by forming concentric holes or coaxial bores in both of the mating pieces; in other words, in the cylinder block and in the main bearing beam or main bearing cap assembly. In fact, a great deal of effort has been expended to employ more than one set of dowels, perhaps as many as eight or ten for a single main bearing cap assembly, while at the same time taking great pains to achieve concentricity between the dowels and the mating holes in the cylinder block. Unfortunately, the inventors have discovered that concentric dowel location is unsatisfactory. A load deflection test was performed on a combination aluminum and nodular iron main bearing cap assembly, or bearing beam, by holding two of the transverse bulkheads or bearing caps steady and by determining the force required to laterally move other bulkheads of the bearing beam. Only a 10-pound force was required to move a bulkhead laterally by a distance of25microns. This demonstrated that the main bearing cap assembly is not in reality a rigid body and can benefit from multiple dowels, say eight for a V-6engine.
The need to maintain concentricity, according to conventional thinking, of course requires very expensive tooling and processing. The present inventors have discovered, however, that by making the dowels and the mating bores off-center, a beneficial interference can be created between the dowels, which are, in a Ford Motor Company design, contained in the main bearing cap assembly, and the mating bores in an aluminum engine block. This offsetting causes an interference to be established between a majority of the dowels and their mating bores, with the result that the main bearing cap assembly may be repeatably and precisely removed from the cylinder block and replaced.
It is therefore an advantage of the present invention that an engine structure according to this invention allows cylinder blocks to be manufactured with precise, repeatable alignment between the main bearing cap assembly and the cylinder block, with the result that engine durability will be increased, reducing engine friction and wear.
Another advantage of the present invention resides in the fact that the force required to complete the engagement between a cylinder block and a main bearing cap assembly according to the present invention is less than the force required with concentric dowel arrangements.
Other advantages of the present invention will become apparent to the reader of this specification.